ChemComm COMMUNICATION Formation of phosphine imide (HNQPH3) and its phosphinous amide (H2N–PH2) isomer† Cite this: Chem. Commun., 2021, 57, 4958 Cheng Zhu, ab Alexandre Bergantini, ‡ab Santosh K. Singh,ab Ralf I. Kaiser, *ab c c d d Received 30th December 2020, Andre´ K. Eckhardt, §* Peter R. Schreiner, Ya-Syuan Huang, Bing-Jian Sun Accepted 12th April 2021 and Agnes H. H. Chang*d DOI: 10.1039/d0cc08411e rsc.li/chemcomm We present the first formation of the previously elusive phosphine imide (HNQPH3) along with its phosphinous amide (H2N–PH2) isomer via exposure of phosphine (PH3) and ammonia (NH3) ices to ionizing radiation. Our approach may be extended to prepare, separate, and detect highly reactive compounds such as intermediates of Wittig reactions. Fig. 1 Computed molecular structures of phosphine imide (HNQPH3, Cs, 1), phosphine oxide (OQPH3, C3v, 2), and methylenephosphorane Since the pioneering discovery by Hermann Staudinger and (H2CQPH3, Cs, 3) at the B3LYP/cc-pVTZ level of theory (ESI†). The atoms Jules Meyer in 1919,1 iminophosphoranes (R1NQPR2R3R4, are color coded in white (hydrogen), blue (nitrogen), gray (carbon), red where R1,R2,R3, and R4 are hydrogen or functional groups) (oxygen), and orange (phosphorous). Bond lengths and angles are given in angstroms and degrees, respectively. have been widely used as key intermediates in the synthesis of primary amines and amides,2–4 chiral nonionic superbases,5 electroluminescent materials in organic light emitting diodes (OLEDs),6 building blocks for PQN-backbone polymers,7 and multiple bonds.16 Comparing 1 to two isomers, phosphinous 8 as ligands for homogeneous catalysis. Particular attention has amide (H2N–PH2, 4)–the mono phosphorus analogue of well- been devoted to the prototype of iminophosphorane–phosphine known hydrazine (H2N–NH2)–and phosphinoammonium imide (HNQPH3, 1) (Fig. 1)–in the theoretical, physical, and ylide (H3NQPH, 5), isomer 1 was computed to be thermo- synthetic chemistry communities from the fundamental view- dynamically more stable than 5 by 0.55 eV (53 kJ molÀ1), but points of chemical bonding and electronic structure.9–15 Electro- less stable than 4 by 0.97 eV (94 kJ molÀ1)(Fig.2andESI†). nic structure and charge distribution analyses revealed that the Nevertheless, a barrier of 2.22 eV (214 kJ molÀ1)forthe PQNbondof1 is highly polar, which is similar to the PQOand hydrogen shift between 1 and 4 suggests that 1 is kinetically PQC bonds of its isovalent species phosphine oxide (OQPH3, 2) stable and can be identified once synthesized. 10,12,13,16 and methylenephosphorane (H2CQPH3, 3), respectively. However, the experimental preparation and characterization The detailed PQX(XQC, N, O) bond properties are still of 1 has remained elusive for more than a century. The contentious, e.g., a resonance hybrid between a dipolar form Staudinger and Kirsanov reactions are the hitherto most used and a double bond form, a resonance between single and synthetic methods for the generation of iminophosphoranes (R1NQPR2R3R4) (Scheme 1).1,17–22 In the Staudinger reaction, a 2 3 4 1 Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, a phosphine (PR R R ) is oxidized by an organic azide (R –N3) Honolulu, HI 96822, USA to produce R1NQPR2R3R4,1,20,21 which can then be hydrolyzed b W. M. Keck Laboratory in Astrochemistry, University of Hawaii at Manoa, to primary amines (Staudinger reaction) or react with a variety 2545 McCarthy Mall, Honolulu, HI 96822, USA. E-mail: [email protected] c Institute of Organic Chemistry, Justus Liebig University, Heinrich-Buff-Ring 17, of electrophiles (aza-Wittig reaction), such as oxiranes, carbon Giessen 35392, Germany. E-mail: [email protected] dioxide, carbon disulfide, isocyanates, carbonyl compounds to d Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, produce aziridines, isocyanates, isothiocyanates, carbodiimides, Taiwan. E-mail: [email protected] 2,4 1 and imines, respectively (Scheme 1). When R –N3 are not † Electronic supplementary information (ESI) available. See DOI: 10.1039/ available, R1NQPR2R3R4 can also be prepared by bromination d0cc08411e 2 3 4 2 3 4 + À ‡ Present address: Centro Federal de Educacao Tecnologica Celso Suckow da of PR R R to bromophosphonium bromide (R R R PBr Br ) 1 Fonseca - CEFET/RJ, Av. Maracana 229, 20271-110, Rio de Janeiro (Brazil). and in situ treatment of the salt with alkylamines (R –NH2) 17,19,22 § Present address: MIT Department of Chemistry, Cambridge, MA 02139 (USA). (Kirsanov reaction). However, the simplest iminophosphorane 4958 | Chem. Commun., 2021, 57, 4958–4961 This journal is © The Royal Society of Chemistry 2021 Communication ChemComm Here we report the first preparation and detection of 1 as well as its isomer 4 by exposing low temperature ice mixtures of PH3 and NH3 to energetic electrons (Table S1, ESI†). Both isomers were unambiguously identified during warming the electron processed ices from 10 K to 300 K at a rate of 1 K minÀ1 (temperature-programmed desorption (TPD)) by selectively photoionizing the isomer based on their ionization energies exploiting tunable vacuum ultraviolet (VUV) single-photon ioni- zation reflectron time-of-flight mass spectrometry (PI-ReTOF-MS). Besides the characterization of the previously absent 1,ournovel route may be extended to form their isovalent species, e.g., 3–the simplest ylide in, e.g., Wittig reactions–which to the best of our knowledge has not been reported either. Fourier-Transform Infrared spectroscopy (FTIR) was utilized to monitor the chemical evolution of the phosphine–ammonia ices induced by the electron processing at 5 K (Fig. S1 and Tables S2, S3, ESI†). The irradiation process decomposed 5 Æ 1% of phosphine and ammonia producing new absorptions À1 À1 of v(PQN) (1100 and 1153 cm ), NH2 (v2, 1508 cm ), v(NRN) À1 À1 À1 (2090 cm ), v(P–H) (2238 cm ), NQNH2 (v5, 2785 cm ), and À1 23–27 v(N–H) (3165 and 3330 cm ). Substitution of ammonia 15 15 (NH3)by N–ammonia ( NH3) shifted the bands of nitrogen- bearing functional groups to lower wavenumbers, which supports the previous assignments (Tables S2 and S3, ESI†). Fig. 2 Potential energy surface for the isomerization of NPH4 isomers at CCSD(T)/CBS//B3LYP/cc-pVTZ including zero-point vibrational energy The identification of new PQN, P–H, and N–H absorptions (ZPVE). Relative energies, point groups, ground electronic states, and could be partly due to the formation of PNH4 isomers. However, computed adiabatic ionization energies corrected for the electric field since electron irradiation can produce a significant array of new effect (À0.03 eV, Stark effect, ESI†) (blue) are also shown (ESI†). The atoms species and isomers with close vibrational absorptions,27 in are color coded in white (hydrogen), blue (nitrogen), and orange (phosphorous). complex mixtures, infrared spectroscopy is able to determine predominantly newly formed functional groups, but does not allow the identification of individual isomers. Therefore, an alternative method was required to identify individual isomers. After the irradiation, the ice was warmed at a rate of 1 K minÀ1 to 300 K; PI-ReTOF-MS was exploited to analyze the sublimed species from the ice.28,29 This method can dis- criminate isomers by systematically tuning the ionization photon energies to selectively ionize the desorbed species with- out fragmentation based on their adiabatic ionization energies (IEs) and then mass resolving the generated molecular ions with the ReTOF-MS. Photon energies of 10.49 eV, 9.43 eV, 8.80 eV, and 8.20 eV were selected based on the computed IEs of the PNH4 isomers (Fig. 2 and Table S4, ESI†). Vacuum ultraviolet (VUV) photons with energies of 10.49 eV and 9.43 eV are able to ionize all isomers (1, 4, 5); 8.80 eV photons can ionize 4 (IE = 8.57 eV), 5 (IE = 6.80 eV), whereas 8.20 eV photons can only ionize 5. Comparison of the signals of the molecular + ions PNH4 (m/z = 49) at these PI energies could lead to the discrimination of the isomers. Fig. 3 compiles the mass spectra of the desorbed molecules from the electron processed ices; the corresponding TPD profiles of the target ions PNH + are Scheme 1 Typical synthetic routes and applications for iminophosphoranes. 4 visualized in Fig. 4. In detail, for the PH3/NH3 system, the signal at m/z =49 HNQPH3 still has not been reported even though the precursors for exhibits a broad sublimation feature with a maximum at 125 K conventional Staudinger and Kirsanov reactions, e.g., phosphine extending from 110 to 160 K at a photon energy of 10.49 eV 15 (PH3), hydrogen azide (HN3), and ammonia (NH3), are easy to (Fig. 4(a)). In a separate experiment using N substituted 15 obtain. nitrogen (PH3/ NH3 system), the peak shifted by 1 amu to This journal is © The Royal Society of Chemistry 2021 Chem. Commun., 2021, 57, 4958–4961 | 4959 ChemComm Communication Fig. 3 PI-ReTOF-MS data during the temperature programmed desorption (TPD) phase of the electron processed ices at distinct photon energies. 15 15 (a), 10.49 eV, phosphine (PH3)/ammonia (NH3). (b), 10.49 eV, phosphine (PH3)/ N-ammonia ( NH3). (c), 9.43 eV, phosphine (PH3)/ammonia (NH3). (d), 8.80 eV, phosphine (PH3)/ammonia (NH3). (e), 8.20 eV, phosphine (PH3)/ammonia (NH3). (f), 10.49 eV, phosphine (PH3)/ammonia (NH3) ice without electron irradiation. (0.4 Æ 0.1):1 (Table S5, ESI†). Deconvolution of the profile at PI = 10.49 eV reveals a 125 K to 132 K ratio of (1.6 Æ 0.1):1, which is significantly higher than that at PI = 9.43 eV; therefore, both peaks are linked to distinct PNH4 isomers as photoioniza- tion cross sections for different isomers vary with respect to photon energies;33 if both sublimation events are linked to the same isomer, the ratios of the integrated ion counts of the sublimation profiles at 125 K and 132 K should be identical at the 10.49 eV and 9.43 eV experiments.